In the field of the photographic industry, for example, there exist a certain number of parameters which are considered as having substantial influence on the uniformity of coating. The principal examples would be the suction (the negative pressure applied close to the point of application of the liquid composition on the support so as to assist the application to the support of the meniscus or liquid bridge formed between the lip of the coating station and the cylinder on which the support is driven), the viscosity of the photographic composition, the speed of movement of the support and the distance between the coating station and the cylinder on which the support to be coated is driven. Non-uniformities of coating result in lines, streaks, bubbles or other defects on the photographic film, substantially affecting the sensitometric properties of the photographic film. Thus, for example, suction is an important parameter in the process of photographic coating. This is because too high a negative pressure will result in a rupturing of the meniscus, while too low a negative pressure will produce an entrainment of air, the two situations being highly detrimental to the uniformity of the photographic coating. It is thus very important to adjust this parameter precisely.
Traditionally, adjustments were made to the photographic coating process parameters on the basis of dry samples of the coated support, which underwent a qualitative evaluation by an operator, and this for a plurality of values of the parameter. The same taking of dry samples and the same qualitative evaluations were carried out while successively varying different parameters of the process. On the basis of all the dry samples thus taken, a "coating map" is produced, on the basis of which the value (or range of values) of each of the parameters studied for which the coating appears most uniform is determined. There are a number of drawbacks with this technique. Firstly, the measurement is supported on a qualitative examination and consequently presents problems of precision. Furthermore, this technique requires a large quantity of dry samples of photographic films to be stored. Finally, this approach does not enable the stability of the meniscus or liquid bridge to be measured, the latter characteristic influencing the uniformity of the coating along the length of the support.